It's luck of the draw, all of my HDD's are pulled from old work PC's some had an easy life, others 24/7. No real pattern which die first.
If they pass SMART then they are fine for retro PC's IMHO. Retro PC's dont get as much use as a daily driver and if/when it does go ok it's an inconvience to reload everything but it's not like you'll loose anything important, maybe a savegame but that's about it.

My 486 has a "lazy" drive for about 2 years now and is on my list of things to do but the total hours I manage to put onto that cant be more then 30 days anually it's still working fine after the drive wakes up.

I backup anything i feel is important, just like i did 30 years ago. If you do the same, I'd run a HDD until it drops. No big loss on an old HDD.

I was thinking about using one of those to move data between systems, and another one to store Wii games. They won't be storing any important data, but...

I've had a drive that completely failed when trying to power on even though it had been running just fine for several years and nothing in S.M.A.R.T. would hint at a major mechanical fault about to happen. So if you're getting hangs and it's not the result of the filesystem driver, you really should attempt to clone the drive ASAP.

Just preference, but I don't play around with unreliable drives. If it starts acting up, I replace it with something else. The biggest hassle, IMO, is taking the case apart and sourcing another drive -- not the loss of data. I like to run real hard drives, but that doesn't mean I trust them. I just use them as long as they're usable.

OTOH, I don't give much weight to SMART statistics. By the time SMART thinks there's something wrong with the drive, there's very obviously something wrong with the drive and I didn't need SMART to tell me. I'll check and see if there were logged errors, and glance at the tabular values for funsies, but I don't bet on them.